1. Field of the Invention
The present invention relates to an optical pickup (optical head) for an optical device such as an optical disk drive, optical card drive, optical scanner, and microscope unit.
2. Description of the Related Art
An optical disk inclusive of a magneto-optical disk has received attention as a memory medium that becomes a core in the recent rapid development of multimedia, and it is usually accommodated in a cartridge case to be provided as an optical disk cartridge for practical use. The optical disk cartridge is loaded into an optical disk drive to perform reading/writing of data from/to the optical disk by means of an optical pickup.
The optical pickup in a recent optical disk drive intended to realize size reduction is composed of a fixed optical assembly and a movable optical assembly, wherein the fixed optical assembly includes a laser diode, a beam splitter for reflecting and transmitting a laser beam, and a photodetector for receiving reflected light from an optical disk, whereas the movable optical assembly includes an actuator having a carriage and an objective lens mounted on the carriage. The carriage is movable in the radial direction of the optical disk along a pair of rails by means of a voice coil motor (VCM).
A write-power laser beam output from the laser diode of the fixed optical assembly is first collimated by a collimator lens, next transmitted by the beam splitter, next reflected by a beam raising mirror of the actuator, and finally focused on the optical disk by the objective lens, thereby writing data onto the optical disk. On the other hand, data reading is performed by directing a read-power laser beam onto the optical disk. That is, reflected light from the optical disk is first collimated by the objective lens, next reflected by the beam splitter of the fixed optical assembly, and finally detected by the photodetector, thereby converting the detected optical signal into an electrical signal.
In general, recording media such as an optical disk and a magneto-optical disk are exchanged for use with an optical disk drive. Further, these recording media have warpage or undulation due to strain in forming the media, resulting in the tendency of eccentricity or inclination of the recording media. Accordingly, focusing error detection and tracking error detection must be carried out to read out information recorded on the recording media. A conventional optical pickup for a magneto-optical disk employs many optical components including a plurality of lenses and a plurality of polarization beam splitters, so as to perform the detection of information recorded on the magneto-optical disk and also perform focusing error detection and tracking error detection.
U.S. Pat. No. 5,708,644 discloses an optical pickup using a beam splitter unit having a polarization beam splitter and a beam splitting element to reduce the size of an optical system. In this U.S. patent, a hologram for separating off a focusing error signal and a tracking error signal from a reflected beam is mounted on the lower surface of the beam splitter unit. Further, a laser diode, a first photodiode for detecting a magneto-optical signal, a second photodiode for detecting the focusing error signal, and a third photodiode for detecting the tracking error signal are mounted on a stem.
Thus, the first, second, and third photodiodes are mounted on the stem in the optical pickup described in the above U.S. patent, so that there is a problem of insufficient integration of the photodiodes. To solve this problem, it is considered to provide a silicon (Si) substrate integrally formed with these photodiodes.
In an optical pickup for a magneto-optical disk, a PIN-photodiode is generally used as each photodiode to meet the requirement for a high-speed response signal. Accordingly, in the case of forming a PIN-photodiode integrally with an Si substrate, a reverse bias voltage is applied to the Si substrate to increase a response speed as a photodetecting element. The application of a reverse bias voltage means applying a bias voltage to the cathode of the photodiode.
The stem (optical base) on which the Si substrate is mounted is bonded to a drive base, so that the stem is at the same potential as a ground potential. Therefore, the lower surface of the Si substrate must be insulated from the stem, so as to apply a reverse bias voltage to the Si substrate. Further, the laser diode chip is mounted on the upper surface of the Si substrate, so that an insulating layer must be interposed between the lower surface (electrode surface) of the laser diode chip and the upper surface of the Si substrate. However, in the case that there is a potential difference between the reverse-biased Si substrate and the electrode surface of the laser diode chip, especially in the case that there are high-frequency variations in potential, the potential of the Si substrate is influenced by variations in potential of the electrode of the laser diode.
Such high-frequency variations in potential occur especially in writing data, and have adverse effects on a photodiode for detection of a focusing error signal, a photodiode for detection of a tracking error signal, and a photodiode for monitoring an output from the laser diode, thus causing instability in detecting signals output from these photodiodes.
A region on the Si substrate except the photodiodes (photodetecting regions) also has sensitivity to light, and generates electrical charge when receiving light. This electrical charge has an influence on signal currents generated in the photodiode regions, causing a problem that high-quality signal currents cannot be obtained. This is due to the fact that all of the light quantities of the laser beam output from the laser diode cannot be transmitted or reflected by each optical component, but a part of the laser beam remains in the optical unit to become stray light.
This stray light may enter the photodiode for detection of a magneto-optical signal, the photodiode for detection of a focusing error signal, the photodiode for detection of a tracking error signal, and a photodiode for automatic power control (APC), causing adverse effects on signal currents. As a known technique for shielding such stray light, a metal film is provided on the entire surface of the substrate except the photodiode regions. The metal film is usually formed of aluminum common to the material of wiring on the substrate. However, reflected light from the optical components is further reflected by the metal film to result in an increase in stray light.